Structure and self-diffusivity of mixed-cation electrolytes between neutral and charged graphene sheets.

Graphene-based applications, such as supercapacitors or capacitive deionization, take place in an aqueous environment, and they benefit from molecular-level insights into the behavior of aqueous electrolyte solutions in single-digit graphene nanopores with a size comparable to a few molecular diameters. Under single-digit graphene nanoconfinement (smallest dimension <2 nm), water and ions behave drastically different than in the bulk. Most aqueous electrolytes in the graphene-based applications as well as in nature contain a mix of electrolytes. We study several prototypical aqueous mixed alkali-chloride electrolytes containing an equimolar fraction of Li/Na, Li/K, or Na/K cations confined between neutral and positively or negatively charged parallel graphene sheets. The strong hydration shell of small Li+ vs a larger Na+ or large K+ with weaker or weak hydration shells affects the interplay between the ions's propensity to hydrate or dehydrate under the graphene nanoconfinement and the strength of the ion-graphene interactions mediated by confinement-induced layered water. We perform molecular dynamics simulations of the confined mixed-cation electrolytes using the effectively polarizable force field for electrolyte-graphene systems and focused on a relation between the electrochemical adsorption and structural properties of the water molecules and ions and their diffusion behavior. The simulations show that the one-layer nanoslits have the biggest impact on the ions' adsorption and the water and ions' diffusion. The positively charged one-layer nanoslits only allow for Cl- adsorption and strengthen the intermolecular bonding, which along with the ultrathin confinement substantially reduces the water and Cl- diffusion. In contrast, the negatively charged one-layer nanoslits only allow for adsorption of weakly hydrated Na+ or K+ and substantially break up the non-covalent bond network, which leads to the enhancement of the water and Na+ or K+ diffusion up to or even above the bulk diffusion. In wider nanoslits, cations adsorb closer to the graphene surfaces than Cl-'s with preferential adsorption of a weakly hydrated cation over a strongly hydrated cation. The positive graphene charge has an intuitive effect on the adsorption of weakly hydrated Na+'s or K+'s and Cl-'s and a counterintuitive effect on the adsorption of strongly hydrated Li+'s. On the other hand, the negative surface charge has an intuitive effect on the adsorption of both types of cations and only mild intuitive or counterintuitive effects on the Cl- adsorption. The diffusion of water molecules and ions confined in the wider nanoslits is reduced with respect to the bulk diffusion, more for the positive graphene charge, which strengthened the intermolecular bonding, and less for the negative surface charge, which weakened the non-covalent bond network.

Structure and self-diffusivity of alkali-halide electrolytes in neutral and charged graphene nanochannels.

Understanding the microscopic behaviour of aqueous electrolyte solutions in graphene-based ultrathin nanochannels is important in nanofluidic applications such as water purification, fuel cells, and molecular sensing. Under extreme confinement (<2 nm), the properties of water and ions differ drastically from those in the bulk phase. We studied the structural and diffusion behaviour of prototypical aqueous solutions of electrolytes (LiCl, NaCl, and KCl) confined in both neutral and positively-, and negatively-charged graphene nanochannels. We performed molecular dynamics simulations of the solutions in the nanochannels with either one, two- or three-layer water structures using the effectively polarisable force field for graphene. We analysed the structure and intermolecular bond network of the confined solutions along with their relation to the self-diffusivity of water and ions. The simulations show that Na and K cations can more easily rearrange their solvation shells under the graphene nanoconfinement and adsorb on the graphene surfaces or dissolve in the confinement-induced layered water than the Li cation. The negative surface charge together with the presence of ions orient water molecules with hydrogens towards the graphene surfaces, which in turn weakens the intermolecular bond network. The one-layer nanochannels have the biggest effect on the water structure and intermolecular bonding as well as on the adsorption of ions with only co-ions entering these nanochannels. The self-diffusivity of confined water is strongly reduced with respect to the bulk water and decreases with diminishing nanochannel heights except for the negatively-charged one-layer nanochannel. The self-diffusivity of ions also decreases with the reducing the nanochannel heights except for the self-diffusivity of cations in the negatively-charged one-layer nanochannel, evidencing cooperative diffusion of confined water and ions. Due to the significant break-up of the intermolecular bond network in the negatively-charged one-layer nanochannel, self-diffusion coefficients of water and cations exceed those for the two- and three-layer nanochannels and become comparable to the bulk values.

Generalized Energy-Conserving Dissipative Particle Dynamics with Mass Transfer. Part 2: Applications and Demonstrations.

We present the second part of a two-part paper series intended to address a gap in computational capability for coarse-grain particle modeling and simulation, namely, the simulation of phenomena in which diffusion via mass transfer is a contributing mechanism. In part 1, we presented a formulation of a dissipative particle dynamics method to simulate interparticle mass transfer, termed generalized energy-conserving dissipative particle dynamics with mass transfer (GenDPDE-M). In the GenDPDE-M method, the mass of each mesoparticle remains constant following the interparticle mass exchange. In part 2 of this series, further verification and demonstrations of the GenDPDE-M method are presented for mesoparticles with embedded binary mixtures using the ideal gas (IG) and van der Waals (vdW) equation-of-state (EoS). The targeted readership of part 2 is toward practitioners, where applications and practical considerations for implementing the GenDPDE-M method are presented and discussed, including a numerical discretisztion algorithm for the equations-of-motion. The GenDPDE-M method is verified by reproducing the particle distributions predicted by Monte Carlo simulations for the IG and vdW fluids, along with several demonstrations under both equilibrium and non-equilibrium conditions. GenDPDE-M can be generally applied to multi-component mixtures and to other fundamental EoS, such as the Lennard-Jones or Exponential-6 models, as well as to more advanced EoS models such as Statistical Associating Fluid Theory.

Generalized Energy-Conserving Dissipative Particle Dynamics with Mass Transfer. Part 1: Theoretical Foundation and Algorithm.

An extension of the generalized energy-conserving dissipative particle dynamics method (GenDPDE) that allows mass transfer between mesoparticles via a diffusion process is presented. By considering the concept of the mesoparticles as property carriers, the complexity and flexibility of the GenDPDE framework were enhanced to allow for interparticle mass transfer under isoenergetic conditions, notated here as GenDPDE-M. In the formulation, diffusion is described via the theory of mesoscale irreversible processes based on linear relationships between the fluxes and thermodynamic forces, where their fluctuations are described by Langevin-like equations. The mass exchange between mesoparticles is such that the mass of the mesoparticle remains unchanged after the transfer process and requires additional considerations regarding the coupling with other system properties such as the particle internal energy. The proof-of-concept work presented in this article is the first part of a two-part article series. In Part 1, the development of the GenDPDE-M theoretical framework and the derivation of the algorithm are presented in detail. Part 2 of this article series is targeted for practitioners, where applications, demonstrations, and practical considerations for implementing the GenDPDE-M method are presented and discussed.

Molecular dynamics of preferential adsorption in mixed alkali-halide electrolytes at graphene electrodes.

Understanding the microscopic behavior of aqueous electrolyte solutions in contact with graphene and related carbon surfaces is important in electrochemical technologies, such as capacitive deionization or supercapacitors. In this work, we focus on preferential adsorption of ions in mixed alkali-halide electrolytes containing different fractions of Li+/Na+ or Li+/K+ and/or Na+/K+ cations with Cl- anions dissolved in water. We performed molecular dynamics simulations of the solutions in contact with both neutral and positively and negatively charged graphene surfaces under ambient conditions, using the effectively polarizable force field. The simulations show that large ions are often intuitively attracted to oppositely charged electrodes. In contrast, the adsorption behavior of small ions tends to be counterintuitive. In mixed-cation solutions, one of the cations always supports the adsorption of the other cation, while the other cation weakens the adsorption of the first cation. In mixed-cation solutions containing large and small cations simultaneously, adsorption of the larger cations varies dramatically with the electrode charge in an intuitive way, while adsorption of the smaller cations changes oppositely, i.e., in a counterintuitive way. For (Li/K)Cl mixed-cation solutions, these effects allow the control of Li+ adsorption by varying the electrode charge, whereas, for LiCl single-salt solutions, Li+ adsorption is nearly independent of the electrode charge. We rationalize this cation-cation lever effect as a result of a competition between three driving forces: (i) direct graphene-ion interactions, (ii) the strong tendency of the solutions to saturate the network of non-covalent intermolecular bonds, and (iii) the tendency to suppress local charge accumulation in any region larger than typical interparticle distances. We analyze the driving forces in detail using a general method for intermolecular bonding based on spatial distribution functions and different contributions to the total charge density profiles. The analysis helps to predict whether an ion is more affected by each of the three driving forces, depending on the strength of the ion solvation shells and the compatibility between the contributions of the charge density profiles due to the ion and water molecules. This approach is general and can also be applied to other solutions under different thermodynamic conditions.

Adsorption of amphiphilic grafted polymers as polymer corrosion inhibitors: insights from mesoscopic simulations.

The homogeneous covering of amphiphillic polymer molecules onto metallic surfaces is of great importance for corrosion inhibitor applications. Lyophillic side chains grafted onto a lyophobic backbone act as anchors that allow the molecule to absorb at the metallic surface preventing the exposure with the solvent. Coarse-grained simulations are used to study the sorption and conformation behaviour of amphiphillic grafted polymers for corrosion inhibition. The backbone insolubility is found to play a key role in the sorption and conformation behaviour in the dilute limit. For finite concentrations, moderate backbone solubility and moderate molecule concentrations achieve optimal surface coverage, while highly a lyophobic backbone leads to bulk-like structures as a consequence of aggregation.

Generalized Energy-Conserving Dissipative Particle Dynamics with Reactions.

We present an extension of the generalized energy-conserving dissipative particle dynamics method (J. Bonet Avalos, et al., Phys Chem Chem Phys, 2019, 21, 24891-24911) to include chemical reactivity, denoted GenDPDE-RX. GenDPDE-RX provides a means of simulating chemical reactivity at the micro- and mesoscales, while exploiting the attributes of density- and temperature-dependent many-body force fields, which include improved transferability and scalability compared to two-body pairwise models. The GenDPDE-RX formulation considers intra-particle reactivity via a coarse-grain reactor construct. Extent-of-reaction variables assigned to each coarse-grain particle monitor the temporal evolution of the prescribed reaction mechanisms and kinetics assumed to occur within the particle. Descriptions of the algorithm, equations of motion, and numerical discretization are presented, followed by verification of the GenDPDE-RX method through comparison with reaction kinetics theoretical model predictions. Demonstrations of the GenDPDE-RX method are performed using constant-volume adiabatic heating simulations of three different reaction models, including both reversible and irreversible reactions, as well as multistep reaction mechanisms. The selection of the demonstrations is intended to illustrate the flexibility and generality of the method but is inspired by real material systems that span from fluids to solids. Many-body force fields using analytical forms of the ideal gas, Lennard-Jones, and exponential-6 equations of state are used for demonstration, although application to other forms of equation of states is possible. Finally, the flexibility of the GenDPDE-RX framework is addressed with a brief discussion of other possible adaptations and extensions of the method.

DPD Modelling of the Self- and Co-Assembly of Polymers and Polyelectrolytes in Aqueous Media: Impact on Polymer Science.

This review article is addressed to a broad community of polymer scientists. We outline and analyse the fundamentals of the dissipative particle dynamics (DPD) simulation method from the point of view of polymer physics and review the articles on polymer systems published in approximately the last two decades, focusing on their impact on macromolecular science. Special attention is devoted to polymer and polyelectrolyte self- and co-assembly and self-organisation and to the problems connected with the implementation of explicit electrostatics in DPD numerical machinery. Critical analysis of the results of a number of successful DPD studies of complex polymer systems published recently documents the importance and suitability of this coarse-grained method for studying polymer systems.

Structural properties of cationic surfactant-fatty alcohol bilayers: insights from dissipative particle dynamics.

Bilayers, self-assembled by cationic surfactants and fatty alcohols in water, are the basic units of lamellar gel networks - creamy formulations extensively used in cosmetics and pharmaceutics. Mesoscopic modelling and study of the bilayers formed by single- or double-tail cationic surfactants (CTAC or DHDAC), and fatty alcohols (FAs) in the lamellar fluid and gel phases were employed. Fatty alcohols with alkyl tail equal to or greater than the surfactant alkyl tail, i.e., C16FA or C18FA and C22FA, were considered. A model formulation was explored with the FA concentration greater than that of the surfactant and the structure of the fluid and gel bilayers in tensionless state characterised via the density profiles across the bilayers, orientational order parameters of the surfactant and FA chains, intrinsic analysis of the bilayer interfaces, and bending rigidity. The intrinsic analysis allows identification and quantification of the coexistence of the interdigitated and non-interdigitated phases present within the gel bilayers. The FA chains were found to conform the primary scaffolding of the bilayers while the surfactant chains tessellate bilayer monolayers from their water-hydrophobic interface. Further, the overlap of the FA chains from the apposed monolayers of the fluid bilayers rises with increasing FA length. Finally, the prevalence of the non-interdigitated phase over the interdigitated phase within the gel bilayers becomes enhanced upon the FA length increase with a preference of the surfactant chains to reside in the non-interdigitated phase rather than the interdigitated phase.

Generalized energy-conserving dissipative particle dynamics revisited: Insight from the thermodynamics of the mesoparticle leading to an alternative heat flow model.

Recently we introduced the generalized energy-conserving dissipative particle dynamics method (GenDPDE) [J. Bonet Avalos, M. Lísal, J. P. Larentzos, A. D. Mackie, and J. K. Brennan, Phys. Chem. Chem. Phys. 21, 24891 (2019)]PPCPFQ1463-907610.1039/C9CP04404C, which has been formulated for an emerging class of density- and temperature-dependent coarse-grain models. In the original work, GenDPDE was formulated to ensure a fundamental link is maintained with the underlying physical system at the higher resolution scale. In this paper, we revisit the formulation of the GenDPDE method, and rederive the particle thermodynamics to ensure consistency at the opposing scale extreme, i.e., between the local thermodynamics in the mesoscopic systems and the corresponding macroscopic properties. We demonstrate this consistency by introducing unambiguous, physically meaningful definitions of the heat and work, which lead to the formulation of an alternative heat flow model that is analogous to Fourier's law of heat conduction. We present further analysis of the internal, unresolved degrees-of-freedom of the mesoparticles by considering the thermodynamics of an individual mesoparticle within the GenDPDE framework. Several key outcomes of the analysis include: (i) demonstration that the choice of the independent variables alters the particle thermodynamic description; (ii) demonstration that the mesoscopic thermodynamic transformations introduce additional terms of the order of the size of the local fluctuations, which prevent an unambiguous definition of both the heat and work; (iii) an emphasis on the importance of the choice of the proper estimators of the thermodynamic properties that are embedded in the chosen thermodynamic description; and (iv) a clearly defined path for determining any thermodynamic quantity dressed by the fluctuations. The further insight provided by this deeper analysis is useful for both readers interested in the GenDPDE theoretical framework, as well as readers interested in the practical ramifications of the analysis, namely, the alternative heat flow model.

Molecular Force Field Development for Aqueous Electrolytes: 2. Polarizable Models Incorporating Crystalline Chemical Potential and Their Accurate Simulations of Halite, Hydrohalite, Aqueous Solutions of NaCl, and Solubility.

The current state-of-the-art force fields (FFs) for Na+ and Cl- ions are not capable of simultaneously predicting the thermodynamic properties of the aqueous solution and the crystalline phase. This is primarily due to an oversimplification of the interaction models used but partially also due to the insufficient parametrization of the FFs. We have devised a straightforward and simple parametrization procedure for determining the ion-ion interaction parameters in complex molecular models of NaCl electrolytes which involves fitting the density, lattice energy, and chemical potential of crystalline NaCl at ambient conditions. Starting from the AH/BK3 and MAH/BK3 FFs, the parametrization approach is employed to develop a complex and accurate polarizable molecular model for the NaCl electrolyte by parametrizing the ion-ion interactions. The performance of the refined polarizable NaCl FF is assessed by evaluating the different thermodynamic and mechanical properties of the crystal, density of crystalline and molten NaCl, along with the melting temperature, properties of aqueous solutions, and the structure and stability of hydrohalite. The simulation results confirm the superiority of the refined FF in comparison with the existing state-of-the-art FFs to accurately predict a wide range of system properties in different NaCl phases, including NaCl aqueous solubility. The refined FF may find applications in the accurate simulations of NaCl electrolytes including inhomogeneous environment, phase equilibria and interfaces, and metastable states. Finally, the parametrization strategy is robust and general and can be used to devise molecular models for other electrolytes.

Generalised dissipative particle dynamics with energy conservation: density- and temperature-dependent potentials.

We present a generalised, energy-conserving dissipative particle dynamics (DPDE) method appropriate for the non-isothermal simulation of particle interaction force fields that are both density- and temperature-dependent. A detailed derivation is formulated in a bottom-up manner by considering the thermodynamics of small systems with the appropriate consideration of the fluctuations. Connected to the local volume is a local density and corresponding local pressure, which is determined from an equation-of-state based force field that depends also on a particle temperature. Compared to the original DPDE method, the formulation of the generalised DPDE method requires a change in the independent variable from the particle internal energy to the particle entropy. As part of the re-formulation, the terms dressed particle entropy and the corresponding dressed particle temperature are introduced, which depict the many-body contributions in the local volume. The generalised DPDE method has similarities to the energy form of the smoothed dissipative particle dynamics method, yet fundamental differences exist, which are described in the manuscript. The basic dynamic equations are presented along with practical considerations for implementing the generalised DPDE method, including a numerical integration scheme based on the Shardlow-like splitting algorithm. Demonstrations and validation tests are performed using analytical equation-of-states for the van der Waals and Lennard-Jones fluids. Particle probability distributions are analysed, where excellent agreement with theoretical estimates is demonstrated. As further validation of the generalised DPDE method, both equilibrium and non-equilibrium simulation scenarios are considered, including adiabatic flash heating response and vapour-liquid phase separation.

Dissipative particle dynamics with reactions: Application to RDX decomposition.

We present a general, flexible framework for a constant-energy variant of the dissipative particle dynamics method that allows chemical reactions (DPD-RX). In our DPD-RX approach, reaction progress variables are assigned to each particle that monitor the time evolution of an extent-of-reaction associated with the prescribed reaction mechanisms and kinetics assumed to occur within the particle, where chemistry can be modeled using complex or reduced reaction mechanisms. We demonstrate our DPD-RX method by considering thermally initiated unimolecular decomposition of the energetic material, cyclotrimethylene trinitramine (RDX), into a molecular gas mixture. Studies are performed to demonstrate the effect of a spatially averaged particle internal temperature and a local reaction volume term in the chemical kinetics expressions, where both provide implicit mechanisms for capturing condensed phase reactivity. We also present an analysis of the expansion of the product gas mixture during decomposition. Finally, a discussion of other potential applications and extensions of the DPD-RX method is given.

Controlled Transport of Flexible Polymers in Slit and Cylindrical Pores Coated with Polymer Brushes: Insight from Dissipative Particle Dynamics.

We use dissipative particle dynamics to simulate the controlled transport of flexible polymers through coated slit and cylindrical pores. Pores are coated inside with solvent-sensitive polymer brushes. Stretch-to-collapse transition then controls the permeability of the coated pores. We change the solvent quality with respect to the polymer brushes and study the flow of flexible polymers through the pores. We show that stretched brush chains close the pores and compress the polymers in the centre of the pores. The collapsed brush chains relieve compression and rapid change in permeability is observed. In open pore state, polymers partially accommodate on the brush layer and partially migrate towards the center of the pores, where the flux is maximal. Finally, we observe that polymers tend to align in the direction of the flow.

Concentrated aqueous sodium chloride solution in clays at thermodynamic conditions of hydraulic fracturing: Insight from molecular dynamics simulations.

To address a high salinity of flow-back water during hydraulic fracturing, we use molecular dynamics (MD) simulations and study the thermodynamics, structure, and diffusion of concentrated aqueous salt solution in clay nanopores. The concentrated solution results from the dissolution of a cubic NaCl nanocrystal, immersed in an aqueous NaCl solution of varying salt concentration and confined in clay pores of a width comparable to the crystal size. The size of the nanocrystal equals to about 18 Å which is above a critical nucleus size. We consider a typical shale gas reservoir condition of 365 K and 275 bar, and we represent the clay pores as pyrophyllite and Na-montmorillonite (Na-MMT) slits. We employ the Extended Simple Point Charge (SPC/E) model for water, Joung-Cheatham model for ions, and CLAYFF for the slit walls. We impose the pressure in the normal direction and the resulting slit width varies from about 20 to 25 Å when the salt concentration in the surrounding solution increased from zero to an oversaturated value. By varying the salt concentration, we observe two scenarios. First, the crystal dissolves and its dissolution time increases with increasing salt concentration. We describe the dissolution process in terms of the number of ions in the crystal, and the crystal size and shape. Second, when the salt concentration reaches a system solubility limit, the crystal grows and attains a new equilibrium size; the crystal comes into equilibrium with the surrounding saturated solution. After crystal dissolution, we carry out canonical MD simulations for the concentrated solution. We evaluate the hydration energy, density profiles, orientation distributions, hydrogen-bond network, radial distribution functions, and in-plane diffusion of water and ions to provide insight into the microscopic behaviour of the concentrated aqueous sodium chloride solution in interlayer galleries of the slightly hydrophobic pyrophyllite and hydrophilic Na-MMT pores.

Chemical potentials of alkaline earth metal halide aqueous electrolytes and solubility of their hydrates by molecular simulation: Application to CaCl2, antarcticite, and sinjarite.

We present a molecular-level simulation study of CaCl2 in water and crystalline hydrates formed by CaCl2 at ambient (298.15 K, 1 bar) conditions and at a high-temperature high-pressure state (365 K, 275 bars) typical of hydraulic fracturing conditions in natural-gas extraction, at which experimental properties are poorly characterized. We focus on simulations of chemical potentials in both solution and crystalline phases and on the salt solubility, the first time to our knowledge that such properties have been investigated by molecular simulation for divalent aqueous electrolytes. We first extend our osmotic ensemble Monte Carlo simulation technique [F. Moucka et al., J. Phys. Chem. B 115, 7849-7861 (2011)] to such solutions. We then describe and apply new methodology for the simulation of the chemical potentials of the experimentally observed crystalline hydrates at ambient conditions (antarcticite, CaCl2·6H2O) and at high-temperature conditions (sinjarite, CaCl2·2H2O). We implement our methodologies using for both phases the CaCl2 transferable force field (FF) based on simple point charge-extended water developed by Mamatkulov et al. [J. Chem. Phys. 138, 024505 (2013)], based on training sets involving single-ion and ion-pair low-concentration solution properties at near-ambient conditions. We find that simulations of the solution chemical potentials at high concentrations are somewhat problematic, exhibiting densities diverging from experimental values and accompanied by dramatically decreasing particle mobility. For the solid phases, the sinjarite crystalline lattice differs from experiment only slightly, whereas the simulations of antarcticite completely fail, due to instability of the crystalline lattice. The FF thus only successfully yields the sinjarite solubility, but its value m = 8.0(7) mol kg-1H2O lies well below the experimentally observed solubility range at 1 bar pressure of (12m, 15m) in the temperature interval (320 K, 400 K). We conclude that the used FF does not provide a good description of the experimental properties considered and suggest that improvement must take into account the crystalline properties.

Adsorption of amphiphilic graft copolymers in solvents selective for the grafts on a lyophobic surface: a coarse-grained simulation study.

The sorption of graft copolymers on surfaces attractive only for the backbone and its effect on the conformational behavior of adsorbed/desorbed chains in solvents good for the grafts and poor for the backbone was studied by coarse-grained computer simulations. It was found that the sorption and conformational behavior are very complex and are results of an intricate interplay of solvent quality (polymer-solvent interactions) and solvent strength (polymer-surface vs. solvent-surface interactions). Increasing grafting density and length of grafts protect the backbone against adsorption, but the behavior is non-trivial. A decrease in solvent quality promotes the adsorption, because it lowers the overall solubility, but the backbone collapses and the probability of backbone-surface contacts decreases, which simultaneously hinders the adsorption. The results of simulations are presented in the form of phase diagrams depicting the decisive features of the conformational and sorption behavior.

Adsorption and Diffusion of C1 to C4 Alkanes in Dual-Porosity Zeolites by Molecular Simulations.

We employ grand canonical Monte Carlo and molecular dynamics simulations to systematically study the adsorption and diffusion of C1 to C4 alkanes in hierarchical ZSM-5 zeolite with micropores (∼1 nm) and mesopores (>2 nm). The zeolite is characterized by a large surface area of active sites on the microporous scale with high permeability and access to the active sites, which arises from the enhanced transport at the mesoporous scale. We model this zeolite as a microporous Na+-exchanged alumino-sillicate zeolite ZSM-5/35 (Si/Al = 35) in which cylindrical mesopores with a diameter of 4 nm have been built by deleting atoms accordingly. We use the TraPPE and Vujic-Lyubartsev force fields along with the Lorentz-Berthelot combining rules to describe adsorbate-adsorbate and adsorbate-adsorbent interactions. The performance of the force fields is assessed by comparing against experimental single-component adsorption isotherms of methane and ethane in microporous ZSM-5/35, which we measured as part of this work. We compare the adsorption isotherms and diffusivities of the adsorbed alkanes in the dual-porosity zeolite with those in microporous ZSM-5/35 and discern the specific behavior at each porosity scale on the overall adsorption, self-diffusion, and transport behavior in zeolites with dual micro/mesoporosities.

Modelling aqueous solubility of sodium chloride in clays at thermodynamic conditions of hydraulic fracturing by molecular simulations.

To address the high salinity of flow-back water during hydraulic fracturing, we have studied the equilibrium partitioning of NaCl and water between the bulk phase and clay pores. In shale rocks, such a partitioning can occur between fractures with a bulk-like phase and clay pores. We use an advanced Grand Canonical Monte Carlo (GCMC) technique based on fractional exchanges of dissolved ions and water molecules. We consider a typical shale gas reservoir condition of a temperature of 365 K and pressure of 275 bar, and we represent clay pores by pyrophyllite and Na-montmorillonite slits of a width ranging from about 7 to 28 Å, covering clay pores from dry clay to clay pores with a bulk-like layer in the middle of the pore. We employ the Joung-Cheatham model for ions, SPC/E model for water and CLAYFF for the clay pores. We first determine the chemical potentials for NaCl and water in the bulk phase using Osmotic Ensemble Monte Carlo simulations. The chemical potentials are then used in GCMC to simulate the adsorption of ions and water molecules in the clay pores, and in turn to predict the salt solubility in confined solutions. Besides the thermodynamic properties, we evaluate the structure and in-plane diffusion of the adsorbed fluids, and ion conductivities.

Flow and aggregation of rod-like proteins in slit and cylindrical pores coated with polymer brushes: an insight from dissipative particle dynamics.

We use a meso-scale dissipative particle dynamics method to simulate the flow and aggregation of rod-like protein solutions through pores grafted with a solvent-sensitive polymer brush. The coated pores can control protein permeability and aggregation by a stretch-to-collapse conformational transition of the brush polymers in response to changes in the solvent quality. The protein solutions mimic aqueous glycoprotein solutions and proteins are represented as rod-like objects formed by coarse-grain beads. The model further employs two types of beads to represent the existence of cystein-like terminal groups in real glycoproteins and mimic the aggregation of real glycoproteins in aqueous solutions. We vary the solvent quality with respect to the brush chains and study the flow and aggregation of rod-like proteins in the slit and cylindrical pores as the brush polymers undergo the stretch-to-collapse transition. The results show that stretched brush chains close the pore, hamper proteins' flow and promote proteins' aggregation. The collapsed brush chains open the pores for proteins' flow and suppress their aggregation. Therefore, we observe more than a ten-fold reduction in the permeation rate of proteins in both pore geometries. Finally, due to pore confinement, larger proteins' aggregates are formed in the slit pore than in the cylindrical pore, while more pronounced orientation of proteins in the flow direction is seen in the cylindrical pore than in the slit pore.